This invention relates generally to semiconductor devices and manufacturing methods, and more particularly to field effect transistors.
As is known in the art, a field effect transistor is often employed to amplify radio frequency power by feeding a radio frequency (RF) voltage signal to a gate electrode thereof for controlling the conductivity of a drain source channel underlying the gate electrode. As is also known in the art, such field effect transistors are often fabricated on semi-insulating crystal substrates of a Group III-V semiconductor material system with gallium arsenide (GaAs) being a common example of one of such material systems. Generally, the crystal substrate is provided with a epitaxially grown semiconductor crystal layer. This epitaxial crystalline layers provide the active regions of the field effect transistor. Radio frequency performance from a field effect transistor is dependent upon the crystalline quality of the semi-insulating substrate end of the epitaxial semiconductor layers used to form the field effect transistor.
Several methods are known for producing crystalline semi-insulating gallium arsenide substrates. One of the most commonly used of such methods is the so-called Horizontal Bridgman technique. This technique generally requires the doping of the substrate with a heavy metal, such as chromium, to compensate for the normal background donor doping of the substrate material to neutralize the donor doping of the substrate and to thus increase the resistivity of the substrate. One problem with chromium doping is that the chromium tends to out-diffuse from the substrate and into the active regions of the transistor formed on the substrate. This outdiffusion of chromium results in a decrease in electron mobility in the so-called drain source channel of the field effect transistor and concomitant therewith degrades the device performance. A second problem is that chromium has an acceptor energy level which is intermediate the so-called valance band and conduction band of the GaAs substrate crystal. At high operation frequencies of the field effect transistor, the rate of recombination of holes and electrons between the valance band of the crystal and the intermediate level of the chromium is lower than the rate of change of an injected current flux in the conduction band resulting in a net fixed negative charge in the substrate. The presence of this negative charge in the substrate repels electrons in the channel region of the device resulting in a loss of gain and loss of device power at those frequencies.
Other methods are also known for growing GaAs substrates. Generally these methods to date, do not provide high quality substrates upon which may be formed active layers for high quality field effect transistors.
One general problem with these methods however, is that in growing substrates of gallium arsenide, for example, interstitial dislocations, and vacancies of gallium or arsenic occur which create electron or hole traps in the crystal that may adversely affect the performance of a field effect transistor fabricated directly thereon. Thus, the crystalline quality of the gallium arsenide substrates is in general relatively poor for fabricating directly thereon, high quality, high frequency field effect transistors.
Another solution in the art to improve the performance of field effect transistors is to provide a semi-insulating epitaxially grown buffer layer over the semi-insulating substrate prior to epitaxial growth of the active layers of the field effect transistor. This buffer layer isolates the active layer of the transistor from the electron or hole traps provided by the substrate due to poor crystal quality. One method generally used in the art to provide such buffer layers is to again use a heavy metal, such as chromium, as a dopant during epitaxial growth. Chromium when used as a dopant provides in the buffer layer the so-called deep acceptor levels mentioned above. The presence of such deep acceptor levels in the buffer layer is generally used to compensate for the normal donor background doping levels encountered during growth of the buffer layer. Thus, a deep level acceptor is used to compensate the buffer layer, as in the case of compensating the substrate, to neutralize the effects of background donor doping in the buffer layer and thus to provide a semi-insulating relatively high resistivity buffer layer. The donor background levels originate from contamination caused by the reaction vessels used to grow the epitaxial layers. As with growing substrates with chromium dopants, a problem with such chromium doping in buffer layers is that in the presence of an RF field some of these deep-acceptor levels will ionize, particularly in substrates which are over-compensated with chromium dopant. Ionization of deep level acceptor states in such a structure produces the layer of negative charge in the buffer layer which will tend to partially deplete the active region underlying the gate electrode. This arrangement, as is the case where chromium doping is used to compensate GaAs substrates, reduces I.sub.DSS, the quiescent drain to source current as well as increases the parasitic resistance associated with the drain-source channel by repelling electrons in the drain-source channel. These effects provide a reduction in both gain and output power particularly at high frequencies. Further, as mentioned above, the chromium is believed to out-diffuse also from the buffer layer and into the active layer resulting in the decrease in electron mobility and loss of power as with out-diffusion from the substrate.
Therefore, while the use of a buffer layer adequately isolates the active regions of the transistor from the effects caused by crystal defects in the substrate, the buffer layer, by being compensated with chromium, does not provide an adequate solution to the out-diffusion and negative charge layer problems.